1. Field of the Invention
The present invention relates to a charged particle beam writing apparatus which includes a stage for mounting a workpiece thereon and beam irradiating unit configured to irradiate with a charged particle beam and which writes a predetermined pattern on the workpiece by irradiating the workpiece with the charged particle beam from the beam irradiating unit.
2. Background Art
The beam irradiating unit in charged particle beam writing apparatuses includes a blanking deflector for selectively allowing or preventing irradiation of the workpiece with a charged particle beam, a shaping deflector for shaping the cross section of the charged particle beam into a predetermined shape, and a main deflector and a sub-deflector for scanning the charged particle beam over the workpiece (see, e.g., Japanese Laid-Open Patent Publication No. 2007-157742). When writing on the workpiece, a write operation is performed on each frame region defined on the workpiece while the stage is being moved, where the frame regions have a strip shape corresponding to the deflection width of the main deflector with respect to the charged particle beam. It should be noted that each frame region is divided into a number of sub-field regions arranged in a matrix fashion. When writing on a frame region, the charged particle beam is positioned at each sub-field region in the frame region by the main deflector and then scanned to each writing position in the sub-field region by the sub-deflector while the irradiation of the workpiece with the charged particle beam is intermittently prevented by the blanking deflector. At that time, the charged particle beam is shaped into a predetermined shape for each writing position by the shaping deflector to write the desired pattern on each sub-field region by irradiating the workpiece with the charged particle beam at each writing position.
It should be noted that the blanking deflector includes a pair of opposite electrodes facing each other across an air gap through which the charged particle beam passes. Further, each of the shaping deflector and the main and sub-deflectors for beam scanning includes a plurality of pairs of opposite electrodes, the opposite electrodes of each pair facing each other across the air gas through which the charged particle beam passes. These opposite electrodes are generally made of a pure metal material. When a high frequency voltage (e.g., 1 GHz) is applied to such opposite electrodes, an eddy current is generated in the electrodes. The magnetic field resulting from the eddy current causes undesirable variation in the deflection angle and direction of the charged particle beam (referred to as “beam drift”), thus lowering the deflection accuracy.
Further, in the blanking deflector, if the distance between the signal line and the ground line is not constant, the resulting characteristic impedance may not be matched to that of the opposite electrodes. Further, the characteristic impedance varies with the frequency of the applied signal. It should be noted that the voltage applied to the opposite electrodes is maximized when the characteristic impedance of the opposite electrodes is equal to that of the cable transmitting the voltage signal to the opposite electrodes, since impedance matching is effected. When their characteristic impedances are not matched, signal reflection, etc. occurs where the signal encounters an impedance change, thereby decreasing the voltage applied to the opposite electrodes. This means that the actual voltage applied between the opposite electrodes varies with the frequency of the applied voltage signal, and as a result, the deflection accuracy is degraded at high frequencies, e.g., 1 GHz.